The increasing availability of Wi-Fi capabilities in wireless devices such as smartphones combined with the free and wide unlicensed spectrum of Wi-Fi networks has led to an increasing use of offloading network traffic onto Wi-Fi networks. Early deployments of Voice over Wi-Fi calling for VoLTE users are now taking market presence.
The mobility of users means that moving between different Radio Access Technologies, RAT, within a single communications session is very common. A problem arises however in that certain parameters related to a communication session set up for on RAT are not optimal when the session is moved to another technology, in particular the codec and the charging regime.
Currently charging differentiation based on the access network used is limited to the type of network in which the call originates. There is no mechanism to apply charging differentiation based on access mobility between Wi-Fi, Long Term Evolution (LTE) and circuit switched legacy networks. This limitation applies to both on-line and off-line charging. There is also no mechanism to trigger a codec change in response to a RAT change.
A possible mechanism to change codec or charging regime would be for the user equipment to send a new Session Initiation Protocol (SIP) INVITE message upon the initiation of a RAT change. However, although this would provide a simple and effective mechanism, there are serious trust issues in allowing the user equipment to initiate a change in charging regime.